A main task of chemical analysis comprises the determination of the composition of an unknown sample material. Thus, by way of example, the molecular formula of a chemical compound is determined in an elemental analysis. In various analysis procedures, especially elemental analysis, it is necessary, in an initial working step, to decompose the substance, that is to be investigated, either completely or partially, into the smallest element-specific molecules or ions. In the case of so-called acidic analysis, this degradation process is carried out with the help of one or more acids, oxidizing agents or other aggressive chemical substances. The fragments that are produced can be determined quantitatively and qualitatively in analysis steps that follow on therefrom. In acidic analysis, it is known that the yield can be significantly improved and the reaction time for the degradation of the substance can be drastically reduced by using high pressures and high temperatures. Thus, in addition to open analysis, analysis is frequently carried out under conditions of increased pressure and temperature. In order to reach the reaction temperatures, use is made of heating apparatus that, for example, utilize thermal radiation, thermal conduction or microwave energy.
The closed analysis vessels cannot withstand any arbitrarily high pressure and, as a rule, they are therefore equipped with a safety valve. The safety valve opens if the internal pressure exceeds a prescribed limiting value in the analysis vessel. During the opening of the safety valve, a portion of the substances, that are contained in the analysis vessel, escapes. An analysis vessel is described in DE 39 19 601 whose internal pressure acts on a pressure recorder that is connected to a control unit for the heating apparatus. As a result, one ensures that the heating apparatus is switched off before the upper pressure limit is reached.
If use is made of several analysis vessels with a communal heating apparatus, then the problem presents itself of monitoring the upper pressure limit for each individual vessel since different pressures arise in the individual analysis vessels especially as a result of exothermic reactions.
In German patent specification DE-OS 38 18 697, an attempt is made to solve the problem by using an individual reference vessel for the measurement of pressure. In the case of different pressures in the analysis vessels, the situation can arise that some of the analysis vessels release their pressure via a safety valve before the upper pressure limit has been reached in the reference vessel and this leads to the switching off of the heating apparatus. This leads to losses of the sample. In the same way, the situation can also arise whereby the upper pressure limit is reached in the reference vessel and the heating apparatus is switched off before the required reaction temperature has been reached in the other vessels.
One possibility for solving the problem of monitoring the pressure in several analysis vessels comprises the feature that each individual vessel is equipped with a pressure measurement device that is connected to the control unit for the heating apparatus. However, this solution is associated with considerable costs.
If microwave energy is used for heating, then a further problem resides in the feature that the electrical lines that are used in, for example, transferring the measurements from the pressure recorder to the control unit for the heating apparatus are capable of absorbing microwave energy or causing interference in a microwave field. This has particularly disadvantageous effects if several analysis vessels are used simultaneously. The microwave field that is used for heating several analysis vessels is then no longer uniform and different amounts of heat are supplied to the individual analysis vessels. Although it is possible to shield the electrical lines with respect to the microwave field, this procedure is nevertheless time-consuming and costly. In addition, resonance positions in the microwave field can generate flash-over phenomena at the connections of the electrical lines.